Aquarium Lighting, Wattage, Kelvin, and Nanometers

Aquarium Lighting, Wattage, Kelvin, and Nanometers

By Carl Strohmeyer


When choosing lighting for your aquarium (especially reef or Nano reef), there is much more to consider than watts per gallon. The 3-4 watts per gallon for a Nano reef or freshwater plant aquarium, less for fish, more for hard coral; is a start but that is very general. There are other factors effecting lighting for your aquarium than just watt output. For example: You cannot compare the output of a 150 watt Metal Halide to a 150 watt outdoor floodlight. What I am trying to say is sometimes it comes down to comparing apples to oranges. In my experience (and admittedly anecdotal), I have found great variations in light bulbs, I have had 20,000K 20 watt bulbs out perform 7500K 36 watt bulbs in my reef aquariums. And the 20,000K 175 watt Metal Halide bulbs have out performed everything I have tried.

Here are two other important factors;

[1] KELVIN RATING (such as 10,000K daylight bulb):

Although I do not believe all the manufacturers are totally honest about true Kelvin output, I believe it should be considered. Kelvin is used to define the heat or energy output of a bulb and if this is applied to two 36 watt PC bulbs, one 6500K the other 10,000K (often daylight or power glow), the 10,000K bulb has a higher energy output. What the true definition of Kelvin is that is a unit of measure of temperature on the thermodynamic (absolute) temperature scale. Here is a brief description of Kelvin: Kelvin is defined by two points: absolute zero, and the triple point of pure water. Absolute zero is defined as being precisely 0 K and –273.15 °C. Absolute zero is where all kinetic energy (motion) in the particles comprising matter ceases, and they are at complete rest. At absolute zero there is NO heat energy. The triple point of water is defined as being precisely 273.16 K and 0.01 °C.

Here are a few Kelvin numbers:

*Absolute zero = 0K (-273.15C)

*Waters freezing point = 273.15 K (0 C)

*Waters boiling point = 373.1339K (100C)


An actinic bulb will have a Nanometer spike at about 420N, a UVC bulb about 265N, a daylight bulb about 700N is used to measure the wave length of light energy from cosmic rays to radio waves. The difference in the wavelength determines how the wave affects its surroundings. It is this wavelength difference that allows short-wave x-ray to pass through walls, while longer-wave visible light cannot pass though the same material; short-wave ultraviolet and x-ray can destroy DNA in living microorganisms and breakdown organic material while visible light will not. Nanometers: Measuring Light Energy All light energy is measured on a “nanometer” (nm) scale. Nanometer means one-billionth of a meter.

This applies to aquariums when we consider the light spectrum and how it applies to our aquariums individual needs: Red light is the first to be filtered out and can only penetrate a short distance. As light waves penetrate deeper into the water, orange and yellow are lost next. Of all the colors of the spectrum blue light penetrates the deepest. Corals need intense equatorial UVA (actinic) and even some UVB as recent articles (and my own experience) suggest. Most plants need more of the infrared spectrum.

The Nanometer scale and Kelvin temperatures come together when applied to aquarium lighting this way; Natural sunlight on a clear day registers at 5500 Kelvin degrees. Kelvin temperatures less than 5500 become more red and yellow and the higher the Kelvin temperature the more blue the light is. Most photosynthetic invertebrates should be kept with lamps of a 20000K rating. Actinic emits a fluorescent blue light and is usually used as supplemental lighting. Not only is actinic lighting beneficial to photosynthetic invertebrates, it is also aesthetically pleasing to the eye when used to supplement “daylight” lighting. Freshwater aquarium plants benefit from lighting with a Kelvin temperature in the range of 5500 – 6500 degrees. Freshwater plants prefer light with more red and yellow in the spectrum. What the exact Kelvin output of an aquarium bulb is takes a little faith in the manufacturer (at least in my opinion), as it is difficult to test each manufacturers claims of Kelvin and the application of Kelvin to aquarium bulbs takes a little bit of scientific stretching (based on the definition of Kelvin).


A basic freshwater fish tank does not need as much lighting and will often do well with one “Aqua Glo”, “Color Max” or similar (30 watt, 350- 750N, 6500K) light for a 60 gallon aquarium. A basic saltwater fish tank also does not have as high of requirements, but more than freshwater (especially if you do not want too much brown algae). A “Coralife 10,000 K” or “Hagen Power Glo” or similar (30 watt, 350- 750N, 10,000K) light for a 60 gallon aquarium.

A freshwater plant aquarium needs more infrared and more watts of light. Four 36 watt with half the bulbs being “Flora Glo” (2800 K 700N +) and half being 10,000K bulbs has worked well for me. Incandescent bulbs have a high infrared output and do well with plants, but also put out a lot of undesirable heat. Cool white fluorescent bulbs should never be used in any aquarium as they only put out visible light and not the spectrum needed by plants or even fish.

A basic reef or Nano reef aquarium usually does well with 50/50 bulbs and/or combinations of 10,000 K, 20,000K and actinic bulbs. In a 10 gallon Nano Reef, two 18 watt Power Compact bulbs will usually do well. Make sure all bulbs (especially in exposed situations common to Nano Reefs are kept clean of water deposits and changed every six months.

An advanced Reef with hard corals needs a combination of metal halide and standard fluorescent or PC bulbs. On a 60 gallon reef aquarium I generally used one 175 watt 20,000K metal halide and one 10,000K daylight (40 watt) and one 6500K actinic (40 watt)

Another point about lighting in general is that higher wave lengths of light such as UVA do not penetrate glass well or even acrylic. I recommend direct lighting (best), quartz or polycarbonate where UVA is essential. Just make sure to clean your bulbs or polycarbonate tops regularly to prevent build up that will block light .

By Carl Strohmeyer

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